1. Field of the Invention
The present invention relates to a black ceramic circuit substrate that possesses the property of opacity to or absorption of light that is used as a circuit substrate for electronic parts such as IC substrate, circuit substrate, and package for integrated circuit.
2. Description of the Prior Art
As the substrate for a ceramic package of an integrated circuit or an electronic display panel in the solid display unit such as desk calculator and counter, use is generally made of high-alumina porcelain.
However, for the use as the substrate, a black substrate is required for the reason of shielding the light in the former case of ceramic package and for the reason of accuracy of digital display in the latter case of electronic display panel. In addition, there is a necessity, for these substrates, of forming numerous patterns for wiring by means of metalization method. Since, however, the metalized surface is obtained by firing or baking a metalizing of Mo-Mn system or the like in a high temperature reducing atmosphere in the electric oven, the constituents of the substrate will also undergo reduction. This leads necessarily to a deterioration in insulation resistance of the substrate so that it was not possible to use black ceramics for the above purpose.
Now, it is well known that a satisfactory black substrate can be obtained by firing Al.sub.2 O.sub.3 as the principal constituent, with addition of several % by weight of SiO.sub.2, MgO, CaO, TiO.sub.2, and Cr.sub.2 O.sub.3, in a reducing atmosphere.
However, when use is made of alumina, the firing temperature is high in the range of 1500.degree. to 1600.degree. C. so that there was a restriction on the conducting materials that may be utilized. For example, in the case of multilayered circuit substrate in which simultaneous sintering is applied to the conducting paste and the substrate, it was necessary to use a conducting paste based on tungsten (W), molybdenum (Mo), and the like that can withstand sufficiently strongly the firing temperature for alumina. However, W and Mo have resistivity of 5.2.times.10.sup.-6 .OMEGA..cm and 5.5.times.10.sup.-6 .OMEGA..cm, respectively, that are about three times as large the value of 1.6.times.10.sup.-6 .OMEGA..cm for Silver (Ag). Accordingly, in order to obtain a conducting line with equal conductivity, it is necessary to make the width of the conducting line to be about three times as large, so that it leads to a drawback that high density wirings cannot be realized.
Moreover, to raise the firing temperature to a high value, the output of the electric oven has to be raised, so that it puts a burden on the electric oven and hence leads to a further drawback of reduction in the life of the electric oven.
Furthermore, there is a disadvantage that satisfactory insulation characteristics may not be obtained unless there is given a strict adjustment on the oxygen partial pressure in the reducing atmosphere.
Still further, although reduction in the firing temperature has been attempted by the addition of glass component, in the case of multilayered wiring, there is generated, due to narrowing of the separation between the wirings, a migration of atoms of metallic conductor in the glass layer so that there is a fear of deteriorating insulation characteristics of the glass layer.
To eliminate these drawbacks there has been desired a development of circuit substrates which enable firing at low temperatures without deteriorating such characteristics as insulation property.